Such a LED light source is known from U.S. Pat. No. 7,081,722. The LED loads are LED arrays comprising series arrangements and possibly parallel arrangements of individual LEDs. The LED light source comprises a rectifier for rectifying the low frequency AC voltage. In case the frequency of the low frequency AC voltage is f, a periodical DC voltage with a frequency 2f and a momentary amplitude varying between zero Volt and a maximum amplitude is present between the output terminals of the rectifier during operation. A series arrangement of the N LED loads is coupled between the output terminals of the rectifier. The LED light source is further equipped with control means for during half a period of the low frequency AC voltage subsequently making the LED loads carry a current, one by one and in dependency of the momentary amplitude of the low frequency AC supply voltage when the amplitude increases and for subsequently making the LED loads stop carrying a current, one by one and in dependency of the momentary amplitude of the low frequency AC supply voltage when the momentary amplitude decreases.
When the momentary amplitude of the periodical DC voltage is zero Volt, none of the LED loads carries a current. When the momentary amplitude of the periodical DC voltage increases, a voltage is reached at which the first LED load starts carrying a current. Similarly, when the amplitude of the periodical DC voltage has increased further to a high enough value, the second LED load starts conducting. Upon further increase of the amplitude of the periodical DC voltage, the remaining LED loads start subsequently to carry a current.
When all of the LED loads carry a current, the momentary amplitude of the periodical DC voltage increases further until the maximum amplitude is reached. After that, the momentary amplitude of the periodical DC voltage starts decreasing. While the momentary amplitude decreases, the LED loads stop conducting a current one by one. After the first LED load has stopped conducting, the momentary amplitude of the periodical DC current decreases further to zero and then the cycle described here-above is repeated.
The known LED light source is very compact and comparatively simple. Furthermore, it can be directly supplied from a low frequency AC supply voltage source such as the mains supply. A disadvantage of the known LED light source is that in the vicinity of the zero crossings of the low frequency AC voltage no light is generated by the LED light source. It is desirable to prevent these light gaps and to thereby prevent stroboscopic effects. A possible solution to this problem is to make use of a “fill in capacitor”. This capacitor is charged in each half period, when the magnitude of the mains voltage is comparatively high and supplies a current to the LED loads when the magnitude of the mains voltage is very low. As a consequence the LED light source generates light continuously. However, a relatively big capacitor is needed, which is undesirable when a flat LED light source is required. Furthermore at least one switching element and a control circuit for controlling the charging and discharging of the capacitor are needed.